This invention relates to measuring machines, and primarily to coordinate measuring machines in which a probe is used to connect a stylus to the head of the machine.
The probes according to the invention are of the digital type in that they provide change from an "off" state to an "on" state when the stylus touches a workpiece. In a conventional manner, the signals from the probe are delivered to the circuitry of the coordinate measuring machine which then records its scale values in the x, y and z directions upon receiving the initial signal from the probe.
Digital probes presently in use suffer from certain inaccuracies which are believed to be avoided by the present invention. In this respect, the accuracy of a digital probe may be defined as the difference in the amount of displacement in all directions required for it to signal contact with the workpiece, plus the amount of variation in its ability to re-seat itself, i.e. return to its "home" or normal rest position.
The most commonly used digital probes require the stylus to pivot away from its normal rest position before a signal is generated. When such pivotal movement occurs, the signal is generated either by a circuit going through the seats which support the stylus in its normal rest position, or by displacing an internal LvDT which creates a trigger voltage.
Since existing probes require some movement of the probe carrier before a signal is provided, they have inherent inaccuracies, and these inaccuracies are not equal for all directions of movement. For example, the phenomenon referred to as EPD, an abbreviation for "effective ball diameter," the workpiece-contacting ball at the tip of the stylus appears to be smaller than it is, simply because the ball must move before a signal is sent to the position recording components of the machine. If the length of the stylus is greater than the radially measured distance between the pivot axis of the probe, the ball size perpendicular to the probe axis is affected more than the size parallel to the axis, thus giving an apparent and inaccurate egg shaped measuring pattern.
Another inaccuracy results when the ball at the tip of the probe must travel further in some directions than in others before a signal is generated. This occurs, for example, when a probe carrier is supported on three seats which are disposed around and spaced from the probe carrier axis. If a measurement is triggered only when the probe carrier becomes unseated at one of the locations, it will be apparent that the amount of movement required to trigger a signal will depend in part on the radial direction at which the probe is moved against the workpiece.
Due to the types of inaccuracies mentioned above, it is a practice in the metrology field for a user first to calibrate the stylus ball while moving in only one direction, and then to perform measurements only when moving in this calibrated direction.
The present invention avoids the inaccuracies described above because a measurement-triggering signal is generated substantially at the moment of contact, i.e. less than one hundred nanoseconds from the instant of contact. This signal occurs before the stylus is displaced. The repeatability of measurements is improved because the probe-positioning seats can be selected purely for suitable hardnesses and friction properties and, because they do not form a component of the electrical trigger switch, they are not subject to electrical arcing.
The invention also provides a number of additional benefit from the standpoint of the simplicity and accuracy of the electrical circuits, the adjustability of sensitivity, the electrical connectors associated with a coordinate measuring machine, and other advantages.